when you have a fleet of 10 or 25.. are their holds full?
It depends what they're buying. If it's Food then yes, they can usually get a full load no matter what the size of the fleet is (unless you get silly and have thousands of merchants). Consumer Goods, Hydrocarbons and Steel depends on the production of the planet. Steel will probably cap at a few 50 ship fleets (depending on distance from your home planet) as it can only reach just shy of 17k on a regular planet. Rare resources (Unobtanium and Krell Metal) will never get full loads simply because of the price to buy them. You may get lucky and fill 1/2 ships, but you're very unlikely to have enough Quatloos on board to buy over 500 of those in a single go.
EDIT: Lost over a million people (1,634,668) on my home planet that turn. Realised that my Quatloos had disappeared so it couldn't have bought food like Dave said it would, likely from all the buying of Food and Steel that it's been doing lately. Put tax rate up to 20% so I can get some cash in. That also means means Long Range Sensors are much too expensive for what they do so I've scrapped them. Next turn I'll build loads of Blackbirds to cover the rest of my PD range around my home planet (like I've done with my secondary main planet).
